A gear oil is a lubricating oil for a gear device, and is used for preventing damages and seizing of gears, for a high-speed and high load gear for an automobile or the like, a relatively low load gear for an ordinary machinery, a relatively high load gear for an ordinary machinery, and the like. The gear oil is generally demanded to be excellent in extreme pressure properties, and particularly, extreme pressure properties are important for a differential gear oil, as compared to MTF (manual transmission fluid). Enhancement in various other properties are also demanded corresponding to the purposes, and various technological developments have been made. For example, Patent Documents 1 and 2 disclose a lubricating oil composition containing a particular ethylene-α-olefin copolymer. The lubricating oil composition is a lubricating oil composition that is excellent in temperature characteristics and is excellent in shear stability. The characteristics that are demanded for a gear oil also include wear resistance, oxidation stability, thermal stability and the like, in addition to the above.
In addition to these capabilities, enhancement of fuel saving properties is demanded for a gear oil for an automobile and the like. As a method for enhancing the fuel saving properties, for example, the use of a gear oil having a low viscosity may be considered for decreasing the viscosity resistance, but the method may suffer shortage of an oil film, which causes additional problems, e.g., deterioration of the seizing resistance and deterioration of the fatigue life of the bearing and gear. It is thus difficult to achieve both the enhancement of fuel saving properties and the basic capabilities of the gear oil simultaneously, and further technological developments have been demanded.